1. Field of the Invention
The present invention relates to a process for producing foamable polyethylene resin particles. In greater detail, the present invention relates to a process for producing foamable polyethylene resin particles containing a polystyrene resin uniformly dispersed therein and having a good molding property which can be pre-foamed to a low bulk density and can form moldings having good rigidity, from which suitable articles can be produced by adding a blowing agent to the polyethylene resin particles to form foamable polyethylene resin particles, heating the particles to produce pre-foamed particles, charging the pre-foamed particles into a mold cavity which permits the escape of gases but retains the foamable polyethylene resin articles, and heating them.
2. Description of the Prior Art
In order to produce foamable polyethylene resin particles which can be pre-foamed to a low bulk density, it is necessary to impregnate a blowing agent consisting of volatile aliphatic hydrocarbons such as n-butane, isobutane, n-pentane, isopentane and dichlorodifluoromethane in polyethylene resin particles. However, although the particles containing the blowing agent can be foamed to a low bulk density just after production thereof, it is necessary to leave the particles for a definite period because the cells produced just after production are very unstable, because polyethylene resin has a high gas permeability. Consequently, there is the disadvantage that particles having a high bulk density can be produced if the particles are allowed to stand, because the blowing agent disappears on standing. Accordingly, methods for producing foamable polystyrene resin particles, for example, a method which comprises suspending polystyrene resin particles in an aqueous medium and adding a small amount of a solvent which dissolves the resin particles and a blowing agent to the resulting suspension to impregnate the blowing agent in the resin particles, and a method which comprises blending polystyrene resin particles and a blowing agent in an extruder, molding by extruding as thin strands, quenching immediately and pelletizing the strands, cannot be applied to the production of the foamable polyethylene resin particles. Even if such methods can be adopted, molding conditions are very severe because the viscoelasticity of the polyethylene at foaming is less than that of the polystyrene resin, and consequently it is difficult to produce preferred articles and moldings.
A process for cross-linking polyethylene resin has been suggested as a means for preventing the disappearance of the blowing agent consisting of the volatile aliphatic hydrocarbons included in the foamable polyethylene resin particles and as a means for adjusting the viscoelasticity at foaming. Namely, a method is known which comprises impregnating the blowing agent using many techniques and cross-linking the polyethylene resin by applying radiant rays before the blowing agent disappears, and, therefore, is lost. According to this method, the cost of the equipment is high, although cross-linking can be carried out in a short period of time. Therefore, this method is not economically preferred, because the foamable polyethylene resin particles produced are expensive. Further it is taught in U.S. Pat. No. 3,743,611 that cross-linking of a polyethylene resin, polymerization of a styrene monomer and permeation of a blowing agent can be simultaneously conducted in a suspension to disperse the polystyrene resin into polyethylene particles uniformly, whereby escape of the blowing agent is prevented and a molding having improved rigidity can be obtained. However, in this method, since cross-linking and impregnation are carried out at the same time, the impregnation of the blowing agent is, of course, carried out at the cross-linking temperature. Accordingly, where n-propane, n-butane, dichlorodifluoromethane, chlorodifluoromethane and dichlorotetrafluoromethane, etc., which are always gaseous at normal temperature and pressure, are used as a blowing agent, the pressure abnormally increases. Thus, if the cross-linking and the impregnation are carried out under such conditions, the polyethylene resin particles are flattened by such high pressures prior to the completion of cross-linking and good foamable polyethylene resin particles cannot be obtained. Further, in order to carry out this method at such high pressures, it is necessary to use a high pressure, high temperature reactor.
Further, since the polymerization of the styrene monomer is carried out in the presence of the blowing agent, there are disadvantages in that the blowing agent causes a decrease in the viscosity of the styrene monomer and consequently a long period of time is required for polymerization, whereby the quantity of the styrene monomer used is limited to below 20% by weight based on the polyethylene resin particles. Further, it simultaneously becomes difficult to control the polymerization and the formation of fine powders becomes high. In addition, the polyethylene resin particles contain polystyrene resin having a low degree of polymerization due to the viscosity decrease, and, as a result, when pre-foamed particles are produced therefrom by heating, cells in the pre-foamed particles become nonuniform or pre-foamed particles having an excellent molding property cannot be obtained, because the second foaming ability thereof is decreased. Finally, articles having excellent rigidity cannot be obtained. Moreover, in the case of using a gaseous blowing agent such as n-propane, n-butane or dichlorotetrafluoroethane, it is necessary to use a high pressure autoclave because the reaction system is under a high pressure.